In conventional vehicles, energy dissipated by braking is lost in form of heat inside the brakes. The energy losses are important because, for example, they can reduce the mileage of an electric vehicle in town traffic of half. Moreover its global energy efficiency is seriously affected.
Several systems for regenerative braking already exist. They can be classified in three families according to their energy storage: battery, capacitor, and flywheel. Some other storage techniques exist but they are not developed or marketed yet.
The power involved in a braking is often much higher than the storage capability whatsoever the storage system in place. Consequently we can usually store only part of the lost energy. For a standard vehicle, the power involved in an emergency braking can exceed 200 KW whereas less than 10 KW can be usually absorbed by a battery under a lot of restrictive conditions regarding its lifetime. Manufacturers mitigate this problem by increasing the battery size to the detriment of the cost and by recovering only a small part of the kinetic energy.
Ultra capacitors can stand much higher power but their storage capacities are low. In addition, this technique is expensive not only due to the capacitors price but also due to the electronic equipment's required to deal with the voltage variations inherent to the capacitors. Moreover, with today practical energy densities of about 2 Wh/Kg, the weight and the volume of the system are far from being negligible for a vehicle.
Flywheels are known for a long time. They have the disadvantage of being heavy, dangerous and they scarcely exceed 2 Wh/Kg. However they appear on some vehicles for competition or public transport in very particular conditions.